U.S. Pat. No. 4,492,276 describes a known drilling system with a known drill bit. The known drilling system comprises an elongate rigid housing with a main central longitudinal axis to which the known drill bit with its own longitudinal axis is coupled such that its axis is under a non-zero tilt angle with respect to the main central longitudinal axis. The drill bit is mounted in a bearing housing on the elongate housing. The elongate housing is provided with a mud motor that can independently drive the drill bit from the rotation of the drill string. A string stabiliser is provided on the top end of the elongate housing and a bearing housing stabiliser is provided near or on the bearing housing relatively close to the drill bit.
For drilling a straight hole, the bearing housing stabiliser and the string stabiliser are activated to engage with the object where the borehole is being drilled in. At the same time as driving the drill bit, the string is rotated thereby driving the elongate housing into rotation independently from the mud motor action. This is called super-imposed rotation while drilling mode. For allowing the elongate housing to rotate, the string stabiliser and the bearing housing stabiliser are both provided with bearings. In sliding drilling mode, the drill string is not rotated, resulting in a curved borehole being drilled by the drill bit as a consequence of it being mounted under an angle and being laterally supported by the bearing housing stabiliser.
When switching from super-imposed rotary drilling to sliding mode drilling ledges may be created in the hole, holding up the assembly and jeopardizing directional performance. In super-imposed rotary drilling mode hole spiralling may cause an over gauge hole to be drilled which may have a drift diameter smaller than the nominal hole size.
In a significantly over gauge hole the bearing housing stabiliser does not contact the hole wall.